A novel application of quantitative phase imaging under linearly polarized light is introduced for studying unstained anisotropic live cells. The method is first validated as a technique for mapping the two-dimensional retardation distribution of a well-characterized optical fiber and is then applied to the characterization of unstained isolated cardiac cells. The experimental retardation measurements are in very good agreement with the established Brace-Köhler method, and additionally provide spatially resolved cell birefringence and phase data.